1. Field of the Invention
The present invention relates to an electronic volume device which controls the sound volume of an audio signal, etc.
2. Description of the Related Art
In general, electronic apparatuses having a function of playing back audio signals, such as CD players, audio amplifiers, car stereo systems, portable radios, portable audio players, etc., include a volume device which allows the sound volume to be adjusted, an equalizer which allows the frequency characteristics to be adjusted, etc. The volume device and the equalizer can be controlled by adjusting the amplitude of the audio signal.
The audio signal is amplified by an amplifier. In the final stage, the audio signal is output as sound via a speaker or headphones provided as an audio output unit. The volume adjustment is attained by controlling the gain of the amplifier or controlling the attenuation factor of an attenuator. For example, electronic volume circuits are disclosed in Patent documents 1 and 2, which allow the gain of an amplifier or the attenuation factor of an attenuator to be adjusted by switching the resistance of a variable resistor.
Furthermore, a final-stage amplifier (which will be referred to as a “power amplifier”), which provides sufficient driving force for driving an electro-acoustic conversion device such as a speaker, headphones, or the like, is provided downstream of the electronic volume circuit. In a case in which an offset (DC offset) occurs in the bias level of the power amplifier, a DC current continuously flows through the speaker. In some cases, this leads to reduced reliability. In order to solve this problem, whether or not such a DC offset has occurred is detected by making a comparison between the output of the power amplifier and a predetermined threshold.
In order to prevent false detection in the playback mode, a technique has been proposed in which the input level of the power amplifier is detected, and the input level thus detected is used as a feedback signal for adjusting a predetermined threshold (Patent document 3).    [Patent Document 1]
Japanese Patent Application Laid Open No. 2005-117489    [Patent Document 2]
Japanese Patent Application Laid Open No. 2005-217710    [Patent Document 3]
Japanese Patent Application Laid Open No. 2004-320273
However, with the technique disclosed in Patent document 3, the level of the input signal of the power amplifier is subjected to AD conversion so as to determine the threshold value with reference to a lookup table. Such an arrangement has a problem of enlarging the scale of the circuit.